Particle accelerators are a key tool for scientific research ranging from fundamental studies of matter to analytical studies at light sources. Cost-for-performance is critical, both in terms of initial capital outlay and ongoing operating expense, especially for electricity to operate the accelerators.
Typically niobium is used to form the superconducting radiofrequency (SRF) accelerator cavities at the heart of many particle accelerators. Presently niobium SRF cavities operate near 1.9 K, well below the 4.2 K atmospheric boiling point of liquid helium to obtain sufficient performance. Thus, significant electric power costs are incurred in operating the SRF cavities at 1.9 K.
More particularly, the BCS surface resistance of niobium at 1.3 GHz decreases from about 800 nΩ at 4.2 K to 15 nΩ at 2 K. The quality factor Q0 (2π times the ratio of stored energy to energy loss per cycle) is inversely proportional to the surface resistance and may exceed 1011. Thus, the strong temperature dependence is the reason why operation at 1.8-2 K is essential for achieving high accelerating gradients in combination with very high quality factors. Superfluid helium is an excellent coolant owing to its high heat conductivity. However, the thermal conductivity of niobium at cryogenic temperatures is strongly temperature dependent and drops by about an order of magnitude when lowering the temperature to ˜2 K.
Niobium nitride is known to have multiple crystalline forms including δ-NbN (a face center cubic (i.e. fcc) crystal form) and ε-NbN (hexagonal close packed (i.e. hcp) crystal form). Nb—N phase diagram was reported quite some years ago. The δ-NbN extends from ˜42 to 50 atomic percent nitrogen and is superconducting. In the prior art, the fcc δ-NbN phase, which is superconducting converts, to the hcp ε-NbN phase upon cooling below 1300° C., and ε-NbN is not superconducting. Furnace nitriding studies have not been able to obtain δ-NbN. Moreover furnace nitriding would require exposing the complete SRF cavity to an aggressive time-temperature history, risking mechanical distortion. Rapid thermal processing with a pulsed heat lamp of Nb films on Si in nitrogen yielded some δ-NbN but a method of preparing a layer substantially composed of δ-NbN and/or on the interior surface of a niobium SRF cavity has not been reported.
The use of a layer of niobium nitride for passivating niobium SRF cavities has been described in U.S. Pat. No. 7,151,347. Niobium SRF cavities are typically fabricated from high purity niobium sheet or cast plate, but often the quality factors at high gradients degrade over time for cavities produced by these methods. Such degradation appears to be affected by adherent surface oxide layers, trapped hydrogen and/or interactions between interstitial oxygen and hydrogen in the niobium material. Passivating niobium cavities with a layer of niobium nitride to reduce the negative effects of these gases and impurities is one approach for addressing these problems. However, the NbN layer obtained in the passivating process is ε-NbN which achieves the passivating objectives but does not contribute enhanced superconductivity properties.
P. Schaaf reported laser gas nitriding of iron, aluminum and titanium, but there appears to have been no work on a solid niobium surface.
Alternatives that would permit operation and sufficient performance of niobium SRF cavities at higher temperatures would be highly desirable.